A. Field of the Invention
The invention relates generally to distribution of messages to a set of communication nodes within a communication network and specifically to distribution of multicast messages to a range of nodes within a network.
B. Explanation of Multicast Message Distribution
An understanding of the problems associated with transmitting multicast messages requires an understanding of networks and nodes. Hence the following explanation is given of relevant terminology and network architecture. Following that explanation is a description of other attempts at multicast message distribution.
1. Terminology
A communication network consists of "communication nodes" and "links." Generally, the term "node" is used to describe a point at which one or more functional units connect to transmission lines. For purposes of this application the term "nodes" will be used to represent the portion of a functional unit in a network which is responsible for communication on that network.
Functional units can include data processing elements designed for communication over a network such as users' data terminal equipment, computer systems running programs which require use of the transmission lines, computer systems dedicated to routing messages within the network or any combination of these possible functional units.
Communication nodes pass messages between each other using different types of communication "links." Communication links are interconnections between communication nodes, point to point connections or local area networks (LANs). Each node in a network is interconnected to at least one other node and is often connected to many nodes by links to those nodes. The nodes pass messages across links connecting the nodes.
A node can communicate "directly" with another node by knowing where that other node is and by forwarding a message to it. A set of nodes in which each node knows the location of the other nodes in the set is called a "group" of nodes. Within a group of nodes each of the nodes can forward a message to another node by forwarding the message through other nodes in the same group to a destination node not directly connected to the sending node.
If the node sends a message to another node which is not in the same group, the transmitting node can communicate "indirectly" with that other node by forwarding a message to a third node or a set of nodes which may then communicate directly with nodes in more than one group. That third node or the set of nodes in turn forwards the message to the node which is the intended destination for the message.
Usually the destination address of a message specifies a single node. In multicast message distribution a "multicast message" is meant to be received by a class of nodes. The destination address of a multicast message includes a "multicast address." A multicast address identifies a class of nodes rather than a single node. Communication nodes are usually configured to process a multicast message differently from a single destination message. Most methods of multicast distribution are inefficient and inflexible.
A multicast message is distributed to all nodes in a "range" of nodes. The multicast range is the set of all the nodes which a multicast message is to be distributed to in the network. Thus multicast message distribution enables a node to deliver a message to a whole class of nodes without knowing the members, and to find one member of a class of nodes.
The intended destination nodes for a multicast message are "logically related" nodes which define a class of nodes. That is, each of the logically related nodes is assigned a common multicast address. Nodes in the network may belong to any number of logically related classes. Therefore, a node may have any number of multicast addresses including no multicast address. A node without a multicast address does not belong to a multicast class of nodes.
When a communication node receives a multicast message, the node compares the multicast addresses of the node and the message and, if any of the addresses match, the node reads the message. Each communication node receiving a multicast message determines, according to a multicast distribution method, to which other nodes the receiving node should forward the message to ensure that all nodes within the multicast range of nodes receives the multicast message.
The range of a multicast message may not contain all of the logically related nodes which are in a class of nodes. Of course, all of the nodes in a range are not necessarily logically related.
Range selection for multicast distribution depends on which set of nodes the node sending a multicast message wants to receive the multicast message. This selection is based on economics and potential location of multicast message destination nodes.
2. Network Architecture
In order to operate a group of directly communicating nodes, each node in the group must "know" the correct directions or links to use in forwarding messages to destination nodes within the group.
In general, knowledge of which link to use in forwarding a message requires the node receiving that message to know the status of its group of directly communicating nodes. The status of those directly communicating nodes includes their current configuration or topology and an identification of which of the directly communicating nodes are currently "active" and may be used to forward a message to another node. An active node is a node capable of communicating to the other nodes in the network.
Knowledge of the status of the group of directly communicating nodes generally requires that each active node in the group know its immediate "neighbors." The neighbors of a node are those active nodes which are connected to that node by a single link.
In one mechanism for inter-node communication to provide such knowledge, each node prepares a packet of information designated as a "link state packet." The link state packet contains, inter alia, the address of the node preparing the packet, the neighbors of the node preparing the packet, and information concerning each link with that node's corresponding neighbors. All of the directly communicating nodes in a group collect these link state packets from other nodes in the group and make a list of the nodes and their associated status information.
The link state packets are primarily used for calculation of pathways through the network. One example of such a calculation which is discussed in A. V. Aho, J. E. Hopcraft, J. D. Ullman, Data Structures and Algorithms, Addison-Wesley, Reading, Mass., 1983, Dijkstra's algorithm, and which is herein incorporated by reference, is the creation of a least cost "spanning tree." The least cost spanning tree is an ordered interconnection of all directly communicating nodes which has no loops and provides the least "cost" path from the node calculating the spanning tree to every other node in a group of directly communicating nodes.
The cost of each path is determined according to criteria chosen by whomever operates the network. These criteria include such factors as the actual price of using the link which could be a leased line, or the volume of traffic through a particular link or node. For example, in order to encourage uniform utilization of the network links, the cost of high volume links would be increased.
Using link state packets, each node determines which nodes and links are active in its corresponding group of directly communicating nodes and then uses that information to determine a spanning tree. The spanning tree of each node dictates which link to use in forwarding messages to directly communicating nodes in the corresponding node group.
The directly communicating communication nodes in each group should each periodically update the status of the other directly communicating nodes by regularly creating and transmitting new link state packets and using the packets received from other directly communicating nodes to update their own spanning tree. This update process ensures, among other things, that if a node becomes inactive, a new operable path connecting every node in the group will be formed; if possible. Updating creates a problem, however. As the number of nodes in a group increases, a greater amount of time is spent calculating the necessary path information. At some point it becomes inefficient to add any more nodes to the group of directly communicating nodes.
Furthermore, greater numbers of nodes in a group require greater amounts of network resources such as processor time required for network operations and computer memory space to hold network status information.
In order to accommodate an increasing number of nodes, a hierarchical structure can be introduced into the network. For example, the network can be divided into several areas, each area can correspond to a different group of directly communicating nodes. Certain nodes will only be able to communicate directly with the other nodes in the same area and these are referred to as "level one nodes", and can be considered to constitute the first hierarchical level.
Other nodes are capable of direct communication with level one nodes in one area as well as similar nodes in other areas. These nodes are referred to as "level two nodes" and constitute a second hierarchical level. All level two nodes communicate with each other and operate in a similar manner to level one nodes communicating in the same area.
Using this scheme, level one nodes within one area may send a message to level one nodes in another area by forwarding the message to a level two node which in turn forwards the message via level two nodes to another level two node within the appropriate area. Once the message arrives in the correct area it is forwarded by level one nodes to the destination node.
3. Prior Attempts at Multicast Distribution
The multicast message must be distributed throughout a range of communication nodes which should include the corresponding nodes for which the multicast message is intended. Thus, every node in the range of nodes which receives the multicast message may be an "interested" node.
One conventional method of multicast distribution is called "reverse path forwarding." In reverse path forwarding, a node first examines a multicast message to determine whether the multicast message is received over a link which the node would normally use in forwarding messages to the node which is the source of the multicast message. If so, the node forwards that message along all of the links connected to the node except the link on which the message was received. In other words, nodes forward multicast messages received on links which correspond to the pathway in its spanning tree which includes the source of the message.
However, this method of multicast distribution is not suited for busy networks because it overloads the network with unnecessary message processing. Unnecessary message processing occurs when a message is received by a node across links not in the spanning tree. Reverse path forwarding causes unnecessary message processing because a multicast message is only accepted for forwarding when the message is from the link in the receiving node's spanning tree corresponding to the message source. This is disadvantageous because a node can receive many copies of the same message which do not require any processing but occupy resources while the node determines if a multicast message should be forwarded. High volume nodes which carry many messages during normal operation should not be burdened by unnecessary message transmissions.
This type of reverse path forwarding is also not suited to hierarchical networks since each node decides whether to forward the message based on the source of the message. In a hierarchical network, this source dependence is not feasible because, inter alia, the hierarchical levels usually are independent of each other and would not necessarily have a spanning tree containing the node.
Modifications to the reverse path forwarding approach of multicast distribution reduce unnecessary message processing but add the expense of computational time and memory space or reduce the system effectiveness. For example, an extension of the basic reverse path forwarding technique involves specifying a distance as a number of links between communication nodes which the message traverses before forwarding is discontinued. However, by limiting the number of links between communication nodes, this method produces an inflexible system which can not adapt to changes in the network topology.
All of these methods of multicast message distribution use network resources inefficiently, are unable to operate in a hierarchical network architecture, and are inflexible.